In an LCD, a Gamma curve and a common voltage Vcom influence the smooth level of the color and the image of the LCD. Since the liquid crystal molecule of the LCD can not be fixed in a voltage for too long, Gamma voltages for driving the liquid crystal molecule divide into the positive pole and negative pole. When the common voltage Vcom is at the center of the positive Gamma voltages and the negative Gamma voltages, i.e. when the common voltage Vcom equals a central value of the Gamma curve, the positive Gamma voltage and the negative Gamma voltage having the same voltage difference with the common voltage Vcom can provide the same gray scale.
FIG. 1 shows a Gamma curve 10 and a common voltage Vcom, in which the Gamma curve 10 is established by a plurality of positive Gamma voltages PV0-PV1023 and a plurality of negative Gamma voltages NV0-NV1023. The plurality of positive Gamma voltages PV0-PV1023 and the plurality of negative Gamma voltages NV0-NV1023 control the gray scales D0-D1023 of an LCD. FIG. 2 shows a circuit that controls the common voltage Vcom, in which an operation amplifier 16 generates and controls the common voltage Vcom. As shown by the waveform 12 in FIG. 1, when the common voltage Vcom is not at the central value 14 of the Gamma curve 10, flickers exist in the image of the LCD. At this time, the common voltage Vcom can be adjusted equal to the central value 14 of the Gamma curve 10 by adjusting a setting signal Vset that is provided to the operation amplifier 16 so as to improve the flicker issue of the image. However, such conventional method for adjusting the common voltage Vcom needs the extra operation amplifier 16. Moreover, the operation amplifier 16 needs a driving current, which causes extra power loss. In addition, due to the bandwidth limitation of the operation amplifier 16, the operation amplifier 16 cannot correct the common voltage Vcom immediately when the common voltage Vcom varies quickly. Further, as shown by the waveform 18 in FIG. 2, the common voltage Vcom provided by the operation amplifier 16 is not fixed but oscillating, and this will cause the flickers of the gray scales, resulting in poorer display performance.